The present invention relates to the production of metal fibers, and in particular to a method of producing metal fibers in a magnetic field and to an apparatus for carrying out the method.
The production of metal fibers or whiskers in a magnetic field by thermal decomposition of metallic carbonyls is known from German Pat. No. 1,224,934. The disclosure of that patent is herewith incorporated herein in its entirety.
According to the teaching of German Pat. No. 1,224,934, carbonyls of metals capable of thermal decomposition are fed into a space in direction counter to a temperature gradient that has been produced in this space. In the heated space the metal carbonyls undergo thermal decomposition and liberate metal atoms which are oriented and stabilized in a homogeneous magnetic field. The patent discloses that it is possible in this manner to produce poly-crystalline metal whiskers or fibers of a length which can be determined and selected at will, and which are characterized by exceptional structural strength. The carbonyls of ferromagnetic materials are fed into an oxygen-free space, which may, for example, be filled with inert gas, in minute quantities on the order of 10.sup.-.sup.4 to 10.sup.-.sup.10 mol per cm.sup.3, counter to a temperature gradient that is produced in this space. The carbonyls undergo thermal decomposition and liberate metal atoms which agglomerate to minute crystals which are oriented by a homogeneous magnetic field with reference to one another and to the flux lines of the magnetic field to produce parallel aggragation chains which are mechanically stabilized by the magnetic field. Thereupon, additional carbonyls of ferromagnetic and/or paramagnetic metals are fed into the space and made to travel in the same counter to the temperature gradient, while at the same time the temperature of the aggregation chains is accommodated to the temperature necessary for a coherent liberation of these latter metals, and the fibers or whiskers continue to grow until they have the desired thickness.
It is possible in this manner to produce metal fibers having a very large ratio of diameter to length, for example 1:100,000 or more, and wherein the diameter of the fiber with reference to the diameter of the coagulation chain has a ratio of 1,000,000:1 or higher. Due to this structural characteristic of the individual metal fibers so produced, metallic materials which are subsequently manufactured from these fibers--e.g., by sintering--can have a strength which heretofore was not attainable.
The apparatus used in this patent for carrying out the method comprises an elongated annular reaction chamber formed between an inner perforated pipe which is heated to the vaporization temperature of the carbonyl and an outer metal pipe which is heated from the exterior by means of heating rods or the like. The inner perforated pipe serves as a vaporizer, and located within it is a further pipe which is heated by means of a circulating heating fluid. On this latter pipe there is located a helical structure on which liquid iron carbonyl seeps downwardly in a spiral path, to undergo vaporization and to enter through the perforated inner tube into the reaction chamber. The upper end of the reaction chamber is closed by a piston which on completion of the operation serves to expel the finished metal fibers from the interior of the chamber into a collecting receptacle that is located beneath the apparatus. Because carbonyls are poisonous, the device is constructed to be gas tight. The annular chamber, that is the interior thereof, is subjected to the influence of a magnetic field which extends parallel to the heating surface, and for which purpose magnetic poles or a magnetic winding are provided. The carbonyl vapor enters the chamber, as already indicated, in direction counter to a temperature gradient produced in the chamber, and the maximum temperature of the heater surface in the region of the outer wall is substantially higher than the temperature at which the metal carbonyls undergo complete thermal decomposition.